Method of removing surface irregularities from metal articles



March 15, 1966 w, BISHOP 3,239,970

METHOD OF REMOVING SURFACE IRREGULARITIES FROM METAL ARTICLES Filed Aug.6, 1962 5 Sheets-Sheet 1 BAUXITE STORAGE F FitIQiliiIT I L C RL[J SE I LCOMMINUTE i F WEEEFUJDW TJ I BAUXITE I L I ..J l|

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INVENTOR.

GEORGE w. BISHOP ATTORNEY METAL LOSS (GRAMS) March 15, 1966 G. w. BISHOP3,239,970

METHOD OF REMOVING SURFACE IRREGULARITIES FROM METAL ARTICLES Filed Aug.6, 1962 5 Sheets-Sheet 2 Fig. 3

METAL LOSS BY THREE MEDIA SINTERED BAUXITE w (AVERAGE OF RUNS a 2) 0.5

0 2o 40 so 80 I00 Tl ME HOURS IN VEN TOR. GEORGE W. BISHOP ATTORNEYMarch 15, 1966 s. w. BISHOP 3,239,970

METHOD OF REMOVING SURFACE IRREGULARITIES FROM METAL ARTICLES Filed Aug.6, 1962 5 Sheets-Sheet 5 MEDIA LOSS NO. 5 PE LLETS N0. 3'; NUGGETS7MEDIA 2o LOSS (PERCENT) s|NTERED BAUXITE PELLETS AVERAGE OF RUNS a 2') 0o 20 40 so 80 I00 TIME HOURS Fig. 4

INVENTOR. GEORGE W. BISHOP ffWM ATTORNEY United States Patent M3,239,970 METHOD OF REMOVING SURFACE IRREGU- LARI-TIES FROM METALARTICLES George W. Bishop, North Tonawanda, N.Y., assignor to TheCarborundum Company, Niagara Falls, N.Y., a

corporation of Delaware- Filed Aug; 6, 1962, Ser. No. 214,887 6 Claims.(Cl. 51--3 13)- This invention relatesto the method. of. removing.surface irregularities from metal articles employing sinteredmicrocrystalline bauxite.

Tumbling media are bodies of various kinds and shapes which are usedforvarious. purposes. Well known tum- 'bling media are porcelain balls andilint pebbles which are placed in a mill together with material to betreated. For instance, porcelain or flint pebbles may be used to mill orgrind porcelain slip or ceramic glaze for tiles. Other grinding mediahave been used for cleaning and polishing metalobjects by tumbling themetal objects and grinding media in a mill.

Grinding media should be suitable for the intended purpose. If it isintended that grinding media beused for removing metal from metalobjects, the rate of metal removal from the object should besatisfactory and the rate of media loss must not be too great. If it istoo great, the cost of the media is excessive.

Grinding media have been made of various materials, one of which is highpurity alumina which is quite expensive. I have discovered that verysatisfactory grinding media can be made from bauxite. Bauxite, inaddition to alumina, contains appreciable quantities of iron oxide,silica and titania. It has been the general belief in the art that thepresence of these oxides prevents the transformation of bauxite intosatisfactory grinding media when bauxite is subjected to hightemperatures. I have found, however, that very satisfactorymicrocrystallinetumbling media can be made from bauxite, rather thanfrom high purity alumina, provided that the process is carried out in amanner more particularly hereinafter described and illustrated in theaccompanying drawings wherein:

FIGURE 1 is a schematic flow diagram of the inventive process, the basicsteps being shown in solid lines to the left and the detailed stepsbeing shown in broken lines to the right;

FIGURE 2 is a perspective view of a typical triangular tumbling pelletproduced in accordance with the invention; and

FIGURES 3 and 4 are graphs illustrating theresults of comparative tests.

A typical analysis of bauxite is:

In carrying out the present method of making microcrystalline bauxitetumbling media, the bauxite, as illustrated in FIGURE 1, is comminutedto a very fine particle size. Substantially all of the: bauxite shouldbe comminuted to an average particle size of not more than 5 microns.Preferably, the majorportion of the comminuted bauxite is less than 1.5microns in size, with not more than 5% greater than 20 microns. Suchsizes may he achieved in a number of'ways. One way is to crush thebauxite to 40 mesh in a roll crusher and then wet grind it in a ballmill, utilizing steel, flint or alumina grinding media, until the propersize is obtained. The bauxite is partially de-watered and mixed with asuitable temporary binder Patented Mar, 15,1966

such as starch or a mixture of starch and methyl cellulose. A smallamount of bentonite, i.e., 1% by weight, may be included in the mixturefor ease of forming.

The aforementioned wet mix is then formed into shapes or bodies whichare to constitute the tumbling media. Although other methods of shapinginto media may be employed, I prefer to extrude amixture of properconsistency through a die and simultaneously cut the extruded rods orcolumn into bodies of the, desired length by use of moving wires orblades.

The mix can be, extruded in triangular form or cylindrical form or in asquare form or any other desired form.

Tumbling media according to this invention may be of various sizes. Thecrossrsectional dimension may vary from about up to about 2", and thelength may be varied according to desire. However, one should allow forabout a 2025% shrinkage which occurs when the extruded bodies are fired.

The extruded bodies are first heated in an oxidizing atmosphere to atemperature sufiicient to burnout the temporary binder; this temperatureshould not exceed 1100 C. and is preferably between a-bout750 to 1000"C. The time required to burn out the temporary binder will depend uponthe particular binder employed, the particular temperature, within theranges above stated, and the method of kiln loading employed for thispurpose.

The bodies are then heated to a higher temperature in order to sinterthem. The sintering atmosphere maybe oxidizing, normal or reducing.Typical firing mixtures are: 20% excess air for oxidation and 2.0%excess gas (viz. propane) for reduction, .based on the normal ortheoretical mixture of gas and air for complete combustion. An oxidizingor normal atmosphere is preferred as an excess of reduction tendstopromote undesirable crystal growth resulting in increased friabilityof the bodies.

The time and temperature during sintering should be carefullycontrolled. If the sintering temperature is not sufiiciently high or thetime of. sinteringis not sufficiently long, the resulting product willnot have adequate strength. On the other hand, if the, temperature istoo high. or the sintering time is too long, an excessive crystal growthmay occur which increases friability. The sintering temperature isbetween 1200 C. and 1600 C., generally be,- tween, 1350 and 1500 C., thepreferred sintering range be.- ing from about 1400 to 1450" C. The. timeof sintering at maximum temperature is between about 2 and 6 hours andis preferably between about 3 and-5 hours. The sin.- te'red bodies arethen allowed to cool at any convenient rate.

The specific gravity of the finished microcrystalline sintered bauxitebodies is about 3.6 to 3.9. The average hardness is about 9+ on the Mohsscal'eand it rangesfrom about 1250 to about 2000 on the Knoop scale.

The use of sintered microcrystalline bauxite tumbling media according tothe present invention offers. considerable savings over the use oftumbling media made from high purity alumina, because it is no longernecessary to use such high purity material to obtain. comparable oreven. better results, as will be demonstrated below.

The cross-sectional shape of the extruded bodies may be. round, square,polygonal, triangular, or of any other desired shape. A preferred shape,however, is triangular, as shown in FIGURE 2. If triangular shapedbodies are formed, the lengths of the sides of the triangle may be from.Ms" up to 2" and the thickness of the bodies may be between /a and Thefollowing example will further illustrate the invention.

EXAMPLE I Surinam bauxite of the composition previously given was ballmilled for 50 hours, usinga wet process; to

produce comminuted bauxite in which substantially all of the comminutedmaterial was less than microns size. The following mix was made:

Parts by Weight Com'minuted bauxite 93.5 Wheat starch 6 Methyl cellulose0.5

About 26 parts by weight of water were used to make an extrudable mass.This was extruded in triangular shapes of approximately 1 /8 on a sideand thick.

The triangular shaped pellets were placed in shallow saggers and placedin a propane gas-fired kiln. A low temperature burn was utilized toremove the temporary binder from the pellets in the initial stage of thefiring process. This was accomplished by increasing the kiln temperatureto 800 C. in increments up to 400 C. per hour, and holding for 4 hours.The temperature was then raised to 1400 C. at a rate of 100 C. per hourand held at that temperature for 2 hours. At this point the firing wasstopped and the pellets were allowed to cool inside the kiln. Thissintering process resulted in an approximate 24% dimensional shrinkageof the pellets. The color was tan to olive on the surface and blackthroughout the inside, the density of the pellets was 3.72 g./ cc.

Tests were run for media loss and metal loss, comparing the pellets madeaccording to Example I with two other types of grinding media, and theresults are shown in FIGURES 3 and 4. In these figures, the pelletsproduced according to Example I are designated as sintered bauxitepellets. One of the other grinding media is designated No. 3 /2 nuggetsand the other grinding media is designated No. 5 pellets.

The No. 5 pellets are triangular shaped aluminum oxide vitrifiedpellets, /1 thick x 1% on each side.

The No. 3 /2 nuggets are aluminum oxide nuggets, generally round inshape, and having dimensions between /2" and /8.

Barrel finishing operations were conducted under the followingconditions.

Media:

(1) 50 lbs. No. 5 pellets (2) 90 lbs. No. 3 /2 nuggets (3) 80 lbs.sintered bauxite pellets (2 batches) Compound: Aluminum oxide abrasivecompound; one

pound per barrel charge Water: /2 gallon per barrel Machine: Four 1.1cubic foot barrels of a standard horizontal barrel finisher (17"diameter x 12" wide) Speed: 150 s.i.p.m. (34-35 rpm.) Metal: 5 pieces of1 in. x 2 in. x /8 in. SAE 1020 steel per barrel Time: 2, 5, 20, 40, 60,80, 100 hours The barrels were charged and run for 2 hours after whichthe metal test pieces were removed and weighed, without dumping themedia, compound and water. This was repeated after 5 hours. For theremainder of the test, starting with 20 hours, the entire contents ofthe barrels were dumped and rinsed, the metal pieces weighed, and freshwater and compound added. The media were dried and weighed after 20, 40and 100 hours.

The results of the tests as shown on FIGURES 3 and 4 show the following:

(1) The media loss of the sintered bauxite pellets after 100 hours was12.9%; the average amount of metal loss on the metal test pieces was2.49 grams (approximately 7.5

(2) The media loss of the No. 5 pellets was 36.8%; metal loss was 2.80grams (approximately 8.5%).

(3) The media loss of 3 /2 nuggets was 20.8%; metal loss was 2.54 grams(approximately 7.7%).

From FIGURES 3 and 4-, it is evident that the sintered bauxite pelletsof the present invention were greatly superior to the other two grindingmedia with respect to media loss and were almost as good as the othertwo media with respect to metal loss.

With respect to the over-all efiiciency of the three grinding media,reference is made to the following Table 1:

Norm-The percent metal loss is obtained by dividing the average weightof the 1 in. x 2 in. x in. metal test pieces into the average weightloss (in grams) of those five pieces. Metal parts weigh approximately 33grams each.

From Table 1, it is evident that the sintered bauxite pellets accordingto the present invention are distinctly superior to the other twogrinding media.

Further tests were run to determine media loss and metal loss, comparingsintered bauxite pellets made according to the present invention withthree high ceramic (silicate) bond aluminum oxide tumbling pellets,types 1, 2 and 3. Type 1 contained 46% bond, 27% grit and finer A1 0 and27% 240 grit and finer A1 0 type 2 contained 50% bond, 25% 150 grit andliner A1 0 and 25% 240 grit and finer A1 0 type 3 contained 70% bond and30% 240 grit and finer A1 0 All of the pellets 1, 2 and 3 and thesintered bauxite pellets were of triangular shape and had the dimensions4; on a side and /2" thick. The test conditions and results were asfollows.

Test conditions The four barrels were loaded with the four differentpellets under the aforementioned conditions. The machine was started andrun for five hours, after which the barrels were dumped of theircontents, the media and metal being washed, dried, and weighed. Thisdata recorded, the pellets were loaded back into their respectivebarrels with fresh water and compound, and run for ten more hours and,finally, for another twenty hours. The results were as follows.

Table 2 MEDIA LOSS [Remaining weightlercent Loss] Type 1 Type 2 Type 3Time, Hours Lbs. Lbs. Lbs. Percent T able 3 1\ IETAL LOSS [Metalremoved-Iercont] Sintered bauxite pellets Time, Hours Type 1 Type 2 Type3 were MUM-4 From the results shown in Tables 2 and 3, it is evidentthat the sintered bauxite pellets according to the present inventionwere distinctly superior to the type 1, 2 and 3 pellets, both withrespect to media loss and with respect to metal loss.

The following Table 4 gives comparative data of applicants sinteredbauxite pellets and a brand of high purity alumina pellets now on themarket:

1 The barrel finishing test consisted oi tumbling the two media for 100hours in separate 1.1 cubic foot closed barrels of ahorizontal tumblerat; 35 rpm. To each barrel was added onclialf gallon of Water, live 1' x2 x it pieces of SAE 1020 steel (for work rate comparisons), and onepound of an abrasive compound. The barrels were rinsed and rechargedevery 25 hours.

It will be seen that for all practical purposes the performance of thetwo grinding media in a simulated normal barrel finishing operation isidentical. It will be noted, however, that, as previously stated, theuse of applicants sintered bauxite grinding media oilers considerablesavings over the use of pellets made of high purity alumina.

I claim:

1. A method of removing surface irregularities from metal articles byabrasion which comprises tumbling said articles with a tumbling mediaconsisting essentially of sintered microcrystalline bauxite.

2. A method according to claim 1 in which said sintered microcrystallinebauxite tumbling media has a hardness of from about 1250 to about 2000on a Knoop scale.

3. A method according to claim 1 in which said sintered microcrystallinebauxite tumbling media has a specific gravity in the range of from about3.6 to about 3.9.

4. A method according to claim 1 in which said sintered microcrystallinebauxite tumbling media has a crosssection dimension of between about and2".

5. A method according to claim 1 in which said sintered microcrystallinebauxite tumbling media is triangular in shape having sides between /sand 2 in length and a thickness of between /8 and 'As 6. A methodaccording to claim 1 in which said sintered microcrystalline bauxitetumbling media has a hardness of from about 1250 to about 2000 on theKnoop scale and a specific gravity of between about 3.6 and 3.9.

References Cited by the Examiner UNITED STATES PATENTS 2,947,124 8/1960Madigan et a1. 51-1645 2,978,850 4/1961 Gleszer 51164.5 3,079,243 2/1963Ueltz 51-309 ROBERT C. RIORDON, Primary Examiner.

JOHN C. CHRISTIE, I. SPENCER OVERHOLSER,

ANDREW R. JUHASZ, Examiners.

1. A METHOD OF REMOVING SURFACE IRREGULARITIES FROM METAL ARTICLES BYABRASION WHICH COMPRISES TUMBLING SAID ARTICLES WITH A MEDIA CONSISTINGESSENTIALLY OF SINTERED MICROCRYSTALINE BAUXITE.